Influence of N, C and S on the initial stage of oxidation of Fe-20Cr single crystals

Fe-20Cr single crystals (100), (110) and (111) have been investigated by Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES). The samples have been oxidised between 500° C and 900° C inside the UHV chamber at oxygen partial pressures between 1×10^–8 mbar and 1×10^–6 mbar. Before the oxidation the samples have been precovered with N, C or S by segregation from the bulk. Oxidation experiments of samples without any impurities on the surface have been carried out for comparison. The oxidation at 500° C results in a preferential growth of Fe-oxide on the samples, with increasing temperature mainly Cr-oxide grows on the surfaces. On S covered surfaces the oxidation is strongly retarded. Presence of N on the surfaces does not change the oxidation behaviour of the samples in comparison to oxidation without N. C on the surfaces retards the oxidation of the surfaces in the very initial stage. During the oxidation the amount of C on the surfaces decreases and no differences to oxidation without C on the surfaces are observed at higher oxygen exposures (100 L). After oxidation no well ordered LEED patterns could be observed, Cr- and Fe-oxide grow in no ordered orientation on the surfaces.